1. Field of the Invention
The present invention relates generally to a phase locked loop, and is directed more particularly to a phase locked loop having an indicating means and a means for selectively adjusting the frequency of a signal derived from a variable frequency oscillator.
2. Description of the Prior Art
A phase locked loop is employed in various electronic instruments, for example, in a stereo demodulator of an FM receiver. When incorporated with an FM stereo demodulator, the phase locked loop is used in conjunction with a circuit for supplying a stereo composite signal to a demultiplexing switching circuit, and comprises a circuit for deriving a pilot signal from the stereo composite signal, a variable frequency oscillator, a phase comparator circuit for comparing the phase of an output signal from the oscillator (or, if preferred, for comparing the phase of a signal which is frequency-divided from the output signal of the variable frequency oscillator) with that of the pilot signal, and a circuit for supplying an output from the phase comparator circuit to the variable frequency oscillator as a control signal. The signal from the variable frequency oscillator which is controlled in response to the output from the phase comparator circuit (or the signal frequency-divided therefrom) is applied to the demultiplexing switching circuit as a so-called switching signal. The switching circuit then separates the FM stereo signal into left and right channel signals.
With the above prior art stereo demodulating circuit, the relationship between the deviation of the phase or frequency of the switching signal fed to the switching circuit and the separation of the stereo composite signal is such that if the phase or frequency of the switching signal varies, for instance, when the stereo composite signal is weak, the separation of left and right channels rapidly deteriorates. In theory, when the loop gain of the phase locked loop is infinite, even if the free-running frequency of the variable frequency oscillator is deviated from its normal frequency, for example, 76 KH.sub.z which is four times of the frequency of the 19 KHz pilot signal, the oscillation frequency of the variable frequency oscillator is locked at 76 KH.sub.z and is also synchronized with the pilot signal, and there is no deviation in the phase and frequency of the switching signal.
In practice, however, since the loop gain of the phase locked loop can not be made infinite, whenever the free-running frequency of the variable frequency oscillator deviates from 76 KH.sub.z, the oscillation frequency led to the switching circuit also deviates. In other words, the deviation of a filtered DC voltage caused by the deviation of the oscillation frequency is balanced with that of the free-running frequency of the oscillator and the circuit stabilizes so that the oscillation frequency of the variable frequency oscillator deviates. When the oscillation frequency of the oscillator deviates, so does the frequency of the signal applied to the switching circuit. As a result, the separation of stereophonic signals into the left and right channel signals by the switching circuit is deteriorated.
To avoid this defect, during manufacture of an FM stereo receiver, the free-running frequency of the oscillator is adjusted at a normal frequency of 76 KH.sub.z to obtain an optimum separation of signals. However, the free-running frequency of this oscillator suffers long-term changes as the receiver ages and short-term, cyclical changes, for instance, as the level of the power supply varies. Further, since the distortion factor and non-linear crosstalk suppression similarly depend upon the phase and frequency of the signal fed to the switching circuit, they are also degraded.